1. Field of the Invention
The present invention relates to a catalyst for producing an .alpha.-olefin comprising a metallocene, an organoaluminum compound, an ionizing ionic compound, and a Lewis base compound. The present invention also relates to a process for producing an .alpha.-olefin polymer employing the above catalyst.
2. Description of the Relate Art
For polymerization of olefins, special active catalysts are known which comprise combination of a cyclopentadienyl derivative of a metal such as titanium, zirconium, and hafnium (Group 4B of Periodic Table) with aluminoxane.
This type of catalyst is not used practically in commercial production of olefin polymers mainly because of the following disadvantages: the aluminoxane cannot readily be synthesized with high reproducibility, and therefore preparation of the catalyst and production of the polymers cannot be conducted with sufficient reproducibility; and aluminoxane is required to be used in a high ratio to the transition metal compound to achieve sufficient catalyst activity although the raw material of aluminoxane, e.g., trimethylaluminum, is expensive.
To offset the disadvantages, ionic metallocene catalysts are reported. JP-A-3-207704 discloses an ionic metallocene compound prepared by reacting a metallocene compound with an ionizing ionic compound. WO92/01723 discloses an .alpha.-olefin polymerization process employing a catalyst system prepared by reacting a halogenated metallocene compound with an organometallic compound and further bringing the product into contact with an ionizing ionic compound, the catalyst system having sufficient catalytic activity.
The catalyst system employing an ionizing ionic compound, which has a high initial activity, loses its activity with progress of polymerization, disadvantageously.
After comprehensive study to solve the above problems, the inventors of the present invention have found that a catalyst prepared by reacting an ionic metallocene catalyst with a Lewis base compound has a stable catalytic species, and exhibits improved productivity of polymers without deterioration of the catalytic activity.